1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to a technique for conveying a sheet while forming a loop.
2. Description of the Related Art
Hitherto, in an image forming apparatus of one of an electrophotographic system and an electrostatic recording system, after a toner image was transferred onto a sheet, the sheet is conveyed to a fixing unit and the toner image is heated, pressed, and fixed by the fixing unit, thereby forming an image onto the sheet.
According to such an image forming apparatus, the sheet onto which the toner image formed by an image forming unit has been transferred passes through the fixing unit, is conveyed to a discharging unit, and is discharged from the image forming apparatus by the discharging unit. Among the image forming apparatuses, there is an apparatus in which after the toner image formed on a photosensitive drum was primarily transferred onto an intermediate transfer belt, it is transferred onto the sheet by a secondary transfer unit.
FIGS. 20A and 20B illustrate an example of a construction of a conventional image forming apparatus. In FIGS. 20A and 20B, a full color image formed on an intermediate transfer belt 131 is transferred onto a sheet P by a secondary transfer unit 130. In the secondary transfer unit 130, toner images of four colors on the intermediate transfer belt 131 are transferred in a lump onto the sheet P by a secondary transfer bias which is applied to a secondary transfer roller 132.
Subsequently, the sheet P onto which the toner images have been transferred as mentioned above is guided by conveying guides 140 and 141 provided between the secondary transfer unit 130 and a fixing roller pair 150. The sheet P is conveyed to the fixing roller pair 150 constructed by a fixing roller 152 and a pressing roller 151. Toner of respective colors is heated and pressed by the fixing roller pair 150, so that the toner is fused, color-mixed, and fixed as a full-color image onto the sheet P. After that, the sheet P on which the image has been fixed is discharged by a discharging roller pair 160 provided downstream of the fixing roller pair 150.
When the sheet P is conveyed from the secondary transfer unit 130 to the fixing roller pair 150, if a sheet conveying speed of the fixing roller pair 150 is higher than that of the secondary transfer unit 130 (secondary transfer roller 132), the sheet P is pulled by the fixing roller pair 150. When the sheet P is pulled, transfer noises (disturbance of an image upon transfer) are generated and a deviation of a copying magnification occurs.
Therefore, hitherto, the sheet conveying speed of the fixing roller pair 150 is set to be slightly lower than that of the secondary transfer unit 130 and a loop is formed in a conveying path 170 between the secondary transfer unit 130 and the fixing roller pair 150, thereby preventing the generation of the transfer noises or the like.
In FIGS. 20A and 20B, a heater 153 is provided in the fixing roller 152. Temperature control is made by the heater 153 so that a surface temperature of the fixing roller 152 is equal to a predetermined fixing temperature.
Quality of the image formed on the sheet P by the image forming apparatus having such a construction changes by one of a heat energy and a pressure energy which are received from the fixing roller pair 150. Particularly, in a color image which is formed by depositing a large amount of toner onto the sheet P, picture quality and glossiness are liable to be influenced by the heat energy and the pressure energy which are received from the fixing roller pair 150 as compared with the case of a black and white (monochromatic) image on which a toner deposition amount is small.
In the case of continuously executing the image forming operation, there is a case where the heat energy lost by the fixing process of one sheet is not sufficiently recovered and the fixing process of the next sheet is started in the state where the surface temperature of the fixing roller 152 decreased. In such a case, the picture quality also deteriorates.
As a countermeasure against the deterioration of the picture quality that is caused by the decrease in surface temperature of the fixing roller 152, hitherto, an electric power which is supplied to the heater 153 is increased (heater capacitance is increased). In this case, when considering an interval of a relatively long time, the supplied energy and the consumed energy can be balanced.
However, even in the case of taking such a countermeasure, if a material such as a heat resistant rubber or the like whose heat response performance is low is used for the surface of the fixing roller 152 in order to improve the picture quality, there is a case where the temperature of the fixing roller 152 decreases temporarily.
When the temperature of the fixing roller 152 decreases as mentioned above, an outer diameter of the fixing roller 152 changes due to such a temperature change. The conveying speed of the sheet P which passes through the fixing roller pair 150 changes. The picture quality and the conveying speed of the sheet also change depending on a difference of the heat capacitance of the sheet P, that is, a sheet type such as thin sheet, thick sheet, OHP film, or the like.
When the sheet conveying speed of the fixing roller pair 150 further decreases due to the temperature decrease of the fixing roller 152, a large loop is formed between the fixing roller pair 150 and the secondary transfer unit 130. When the large loop is formed as mentioned above, particularly, in the case of the image forming unit in which a distance between the secondary transfer unit 130 and the fixing roller pair 150 is small, as shown in FIG. 20B, the image surface of the sheet P is come into contact with the conveying guide 141 and is rubbed, so that a defective image and the transfer noises (disturbance of the image upon transfer) occur.
To solve such a problem, there is an apparatus in which the sheet conveying speed of the fixing roller pair (fixing unit) 150 is controlled by the first speed lower than the sheet conveying speed of the secondary transfer unit (transfer unit) 130 and by the second speed higher than the first speed.
When the sheet conveying speed of the fixing roller pair 150 is controlled by the first speed, in the case where a height of the sheet loop which is formed between the fixing roller pair 150 and the secondary transfer unit 130 is equal to a predetermined height or more, the speed is switched to the second speed. When the height of the sheet loop is equal to the predetermined height or less, the sheet conveying speed of the fixing roller pair 150 is switched to the first speed. In this manner, a size of sheet loop which is formed between the fixing roller pair 150 and the secondary transfer unit 130 is maintained within a predetermined range.
The techniques of switching the sheet conveying speed as mentioned above have been disclosed in Japanese Patent Application Laid-Open Nos. H05-107966, H07-234604, H10-097154, 2000-344385, 2001-282072, 2003-345150, and 2006-23655.
An example of the loop control will now be described with reference to FIGS. 20A and 20B. A flag 162 is arranged in the conveying path 170 between the secondary transfer unit 130 and the fixing roller pair 150. When the sheet P forms a loop, the flag 162 rotates around a rotary axis 161 as a rotational center by a pressure of the sheet P. A rotational angle of the flag 162 which changes depending on the loop of the sheet P is detected by a sensor 163. The loop control at the first speed and the second speed of the fixing roller pair 150 is made based on a detection signal from the sensor 163, thereby stabilizing the loop of the sheet P.
In the conventional image forming apparatus in which such a loop control is made, it is presumed that the loop which is formed in the sheet P between the transfer unit and the fixing unit has a loop shape in such an orientation that the surface on which the non-fixed toner of the sheet P is not transferred becomes a convex surface (hereinafter, such a loop is called a normal loop) as shown in FIG. 20A. That is, by inclining a nip direction of the secondary transfer unit 130 and a nip direction of the fixing roller pair 150 at a predetermined angle, the normal loop in such an orientation that the non-image surface side of the sheet P becomes a convex surface is formed.
However, in the case of a sheet on which a large amount of toner has been deposited upon creation of the image of the first plane, for example, in a duplex image forming mode, there is a case where the sheet is largely curled so as to become a concave shape toward the toner surface side after the fixing. In such a case, upon image creation of the second plane, there is a case where the sheet is conveyed in the state where the sheet is curled so that the surface on which the non-fixed toner has been transferred becomes a convex surface between the transfer unit and the fixing unit. When the sheet P is left in a high-temperature and high-humidity environment, there is a case where a large curl is caused by a difference between a moisture absorption situation of the moisture on the obverse surface of the sheet and that on the reverse surface. Depending on the obverse/reverse side of the curled sheet P which is set onto a sheet feeding tray, there is a case where the sheet is conveyed in the curled state so that the surface on which the non-fixed toner has been transferred becomes the convex surface between the transfer unit and the fixing unit.
After the image was transferred by the secondary transfer unit 130 onto the sheet in which such a curl that the surface on which the non-fixed toner is transferred becomes the convex surface had occurred as mentioned above and the sheet entered the fixing roller pair 150, if the loop control is made, there is a case where the sheet enters a state as shown in FIG. 20B. That is, such a loop shape that the surface on which the non-fixed toner has been transferred becomes the convex surface (hereinafter, such a loop is called a reverse loop) occurs suddenly. If such a reverse loop occurs, since the flag 162 does not light-shield the sensor 163, the sensor 163 cannot detect the sheet P. Therefore, since the detection signal is not input from the sensor 163, a control unit (not shown) erroneously determines that a loop amount of the sheet P is small.
If such an erroneous determination is made, the control unit continuously supplies a signal instructing that the sheet conveying speed of the fixing roller pair 150 is set to the first speed lower than that of the secondary transfer unit 130 so as to further increase the loop amount. Thus, an amount of reverse loop increases and the image surface side of the sheet P is soon come into contact with the guide 141 on the side opposite to the sensor 163. The non-fixed toner on the sheet is rubbed. Thus, the defective image of the sheet occurs.
According to the invention disclosed in, for example, Japanese Patent Application Laid-Open No. 2006-23655, a sheet attracting unit for attracting the sheet in the orientation of the loop which is detected by a loop detecting unit is provided between the transfer unit and the fixing unit and an attracting force of the sheet by the sheet attracting unit is set to a value within a range where the reverse loop of the sheet can be eliminated.
However, when presuming the case where a large curled sheet whose image surface side becomes the convex surface is conveyed, an apparatus whose attracting force is very large has to be arranged in order to eliminate the reverse loop, resulting in an increase in size and costs of the apparatus. Moreover, if the attracting force is set to be too large, there is also a case where the toner in the apparatus is scattered, the sheet itself vibrates, and a trouble of the defective image occurs. Further, in order to solve such a problem, if the transfer unit and the fixing unit are arranged so that the image surface side is difficult to become the convex surface, there is such a problem that a degree of freedom upon designing of a layout construction of the conveying path and the whole apparatus decreases.